In the chemical industry, the hydrogenation of carbonyl compounds, in particular carboxylic acids or carboxylic esters, with the aid of heterogeneous catalysts plays an important role. In principle, both slurry processes and fixed-bed processes are possible for these hydrogenations, with the fixed-bed processes predominating. In the slurry process, the catalysts are used as powders, while shaped catalyst bodies are used in the fixed-bed process.
The hydrogenation of carbonyl compounds is carried out using, in particular, catalysts containing Ni, Cu, Co or noble metals. These can be used as all-active catalysts (e.g. Raney catalysts) or as supported catalysts.
The patent documents DE 43 45 265 and DE 43 35 360 describe shaped Raney catalysts based on Ni, Co, Cu and Fe. These are used for hydrogenating organic compounds. The disadvantage of these catalysts is the addition of metal powders as binders, with the metal powder added being less catalytically active than the Raney metal.
The production of shaped Raney catalysts without addition of binders is described in EP 880 996. These catalysts are used for hydrogenating nitriles. To produce these catalysts, a metal-aluminum alloy present as powder is mixed with a high molecular weight polymer and optionally promoters and subsequently shaped to give shaped bodies. The shaped bodies are calcined at temperatures of up to 850° C., which leads to controlled decomposition of the polymer and formation of a fixed-bed catalyst having sufficient mechanical stability. Activation is effected by leaching out of the aluminum by means of sodium hydroxide solution. However, the leaching out of the aluminum and thus activation of the catalyst occurs only in the outer shell of the shaped body. The core of the catalyst continues to consist of the metal-aluminum alloy used and serves as support for the activated outer layer of the catalyst. As a result, a considerable part of the relatively expensive alloys remains unutilized.
Apart from the Raney catalysts, substantially Cu and Ni catalysts supported on various metal oxides such as Al2O3 or SiO2 are used for the hydrogenation of carbonyl compounds.
Thus, for example, U.S. Pat. No. 4,666,879 describes an extruded copper chromite-aluminum oxide catalyst produced by mixing of from 40 to 82% by weight of copper chromite and from 18 to 60% by weight of aluminum oxide. The Al2O3 is typically used in the form of pseudoboehmite or hydroxyboehmite. After calcination, the extruded catalyst is suitable for the liquid-phase and gas-phase hydrogenation and hydrogenolysis of various carbonyl compounds and functional side groups of aromatic compounds. The BET surface area of the extruded catalysts is typically in the range from 125 to 225 m2/g.
U.S. Pat. No. 4,762,817 describes a catalyst for the hydrogenation of aldehydes, which consist essentially of a mixture of copper and zinc oxide. An improvement in the selectivity was able to be achieved by impregnation with alkali metals such as sodium, potassium, lithium or cesium, in combination with a transition metal such as nickel, cobalt or mixtures thereof.
The U.S. Pat. No. 4,929,777 describes catalyst compositions containing oxides of Cu and Ti and the use of such catalyst compositions in the hydrogenation of particular esters to the corresponding alcohols.
The U.S. Pat. No. 5,008,235 describes a process for hydrogenating organic aromatic or nonaromatic acids and esters thereof to give the corresponding alcohols using a coprecipitated catalyst. The catalyst contains copper, aluminum and a further metal such as magnesium, zinc, titanium, zirconium, tin, nickel, cobalt or mixtures thereof and is subjected to reduction before use. The temperature in the reduction is increased stepwise up to a final temperature of from 150° C. to 250° C.
The U.S. Pat. No. 5,093,534 describes a two-stage process for hydrogenating saturated and unsaturated aldehydes to alcohols using Cu- and Ni-containing catalysts. The first stage of the hydrogenation is carried out using a particulate copper catalyst which has been made alkaline. In the second stage of the hydrogenation, a supported nickel-containing catalyst whose support material has acidic sites having a particular acid strength is used.
The U.S. Pat. No. 5,124,295 describes an extruded copper chromite catalyst consisting of a mixture containing from about 20 to 80% by weight of copper chromite and from about 20 to 80% by weight of an extrudable inorganic binder. The catalyst has a specific surface of from about 20 to 225 m2/g and the total pore volume of the pores in the catalyst is from 0.35 to 1 cm3/g. In one embodiment, this document describes a process for producing a shaped copper chromite catalyst by producing an extrudable mixture, extruding the mixture and calcining the extrudate. The catalysts are employed for the hydrogenation of aldehydes, ketones, carboxylic acids and carboxylic esters.
The U.S. Pat. No. 5,134,108 describes a hydrogenation catalyst comprising oxides of a first metal, copper or zinc, and a second metal, chromium, molybdenum, tungsten or vanadium, and optionally an oxide of a promoter such as manganese, barium, zinc, nickel, cobalt, cadmium or iron. The hydrogenation catalyst is present as a powder having an average particle diameter of from about 6 to 20 μm and a surface area of from about 20 to 70 m2/g. The catalysts are produced by precipitation of the metal salts by means of a base.
U.S. Pat. No. 5,155,086 and U.S. Pat. No. 5,345,005 describe a pulverulent catalyst which consists of a major part of the oxides of copper and zinc and to a smaller part of aluminum oxide, with the atomic ratio of copper to zinc being from 0.2 to 5.5. The catalyst is produced by precipitation, e.g. at a pH of >7, and calcination of the precipitate. The hydrogenation catalysts are used for the hydrogenation of aldehydes, ketones, carboxylic acids and carboxylic esters.
WO 92/04119 describes copper-manganese catalysts for hydrogenating fatty acids and esters thereof. They are produced by admixing an aqueous solution of Cu(II) and Mn(II) salts with sodium hydroxide to form a precipitate of Cu hydroxide and Mn hydroxide. This precipitate is then calcined as powder or in tableted form. The catalysts obtained have a BET surface area of from about 3 to 45 m2/g.
WO 02/47818 describes catalysts containing Cu oxide for hydrogenating maleic anhydride and derivatives thereof. As pore formers, graphite and ammonium nitrate, in particular, are used here and are mixed into the catalyst powder before tableting. The catalysts for which exclusively graphite has been used as pore former in the production of the catalysts had a pore volume of less than 0.2 cm3/g.
WO 97/34694 describes copper oxide/aluminum oxide hydrogenation catalysts which are produced by precipitation of aqueous solutions of copper nitrate and sodium aluminate by means of sodium carbonate. The material obtained is dried and then calcined at from about 400° C. to 700° C. and subsequently tableted with addition of graphite. The pellets have a pore volume of from 0.2 to 0.6 ml/g and a bimodal pore radius distribution having a first maximum at about 10 nm and a second maximum at from about 50 to a maximum of 200 nm.
In the commercial use of catalysts, an increase in the conversion into the target product is of particularly great interest with a view to achieving a further improvement in the economics.
The conversion of a catalyzed reaction is determined by, inter alia, the activity of a catalyst which in turn is influenced by, inter alia, the magnitude of the “metal surface area” of the catalyst. In general, the term “metal surface area” refers to the accessible surface area of the active component of the catalyst. The catalyst is generally converted into the catalytically active metallic state by reduction of an oxidic precursor of the active component (for example copper in the oxidic form CuO is converted by reduction of Cu(II) to Cu(0)) before actual use. A high Cu metal surface area is associated with a high activity. In general, thermal treatments always lead to more or less pronounced sintering of the metal particles. Sintering reduces the accessible surface area of the active component of the catalyst. Since the reduction of the catalyst usually takes place at elevated temperature, suitable temperature conditions have to be observed in order to keep the sintering effects as small as possible.
In the light of this background, it is an object of the present invention to provide a process for producing tableted catalysts which in the reduced state have a relatively high metal surface area, determined by N2O pulse chemisorption, compared to catalysts of the prior art.
A further object is to provide tableted catalysts which have a higher activity in hydrogenation reactions than do catalysts of the prior art.
This object is achieved by the process of the invention and the catalysts obtainable thereby.